Possible Clues:

• Culprit in California quakes
• Cause of many California earthquakes
• Safe natural sand is moving in area of instability
• Source of some serious shaking
• What might shift under LA, SF and Santa A?
• This could cause a major shake-up in Hollywood
• Source of some large-scale shaking
• Site of slippage ... both geographically and in this puzzle
• California woman's imperfection?
• Meeting place of California's Plates Movement